It is known to use integrating amplifiers (integrators) as low pass filters in applications in which small amplitude symmetrical and repetitive or periodic AC signals must be processed and measured with precision. One such application is in connection with Coriolis effect-type meters such as that shown by Samson and Zolock in U.S. Pat. No. 4,843,890 issued Jul. 4, 1989. In this patent, small amplitude AC signals generated by sensors associated with the Coriolis meter are amplified and applied to integrators which filter the high frequency noise and apply the filtered signals to a logic circuit. The logic circuit measures the time delay (.DELTA.t) between the filtered signals to determine the mass flow rate of the material flowing through the meter. The accuracy of the measurement performed by the logic circuit depends on an accurate measurement of the time delay between the signals applied to the logic circuit. Therefore, the signal processing circuitry intermediate the Coriolis effect meter and the logic circuit must not introduce any random time delays in the signals applied to the logic circuit. For this reason, the processing circuitry is DC coupled to prevent the generation of any undesired time delays. It is also desirable that the input signals be as free from noise as is possible. Samson and Zolock disclose integrators that operate as low pass filters to remove high frequency noise from the signals and to apply to the logic circuitry noise-free signals having the same time delay therebetween as do the two pickoff signals generated by the Coriolis effect meter.
Since the amplifier stages preceding the integrators are DC coupled to the integrators, it is possible that the AC signals applied to the integrators may include a DC component produced by the amplifiers to which the inputs of integrators are DC coupled. Also operational amplifiers used as integrators also generate offset voltages in their operation which have similar detrimental effects. The voltage gain of integrators at low frequencies or at DC is relatively large. Therefore the resulting DC components of input signals can unbalance the integrators and adversely affects their operation. This, in turn, can degrade the operation of the logic circuit that measure the time delay between the integrator output signals.
Attempts have been made to remove this DC component. However, these attempts have been successful only to a limited extent and all have accompanying disadvantages. One such prior art solution is to periodically short the input and output of the integrator. This is undesirable in some applications since it interrupts the operation of the integrator while its input and output are short-circuited. It is further undesirable since the DC component in the output signal reappears immediately upon the short circuit being removed. Another attempted solution is the use of a DC feedback amplifier connected between the output and the input of the integrator. The feedback amplifier receives a filtered portion of the output of the integrator and applies this signal back to the input of the integrator to cancel the DC component. This expedient is not ideal. The input of the feedback amplifier includes a frequency selective network which filters out the AC component of the integrator output signal and applies only a DC signal to the input of the feedback amplifier. This low pass filter is necessary so that the feedback amplifier does not feed back any AC signal from the output to the input of the integrator.
This low pass filter has a low corner frequency of about two to five Hertz (Hz). While this low corner frequency permits the amplifier to function as a DC feedback amplifier, it is disadvantageous in that with this corner frequency of two to five Hz can momentarily unbalance the operation of the integrator when the integrator receives a signal having a frequency component within and below this frequency range. In this case, the integrator operates as a high gain amplifier and generates a large amplitude damped output signal in the two to five Hz range. This damped signal affects disadvantageously the operation of the logic circuit. In addition, the low pass filter increases the response time of the flow metering device, producing slow responses to fast process changes.
The use of the DC feedback amplifier is further disadvantageous since the low pass filters in the right and left channels of a Coriolis meter system may have slightly different frequency responses that can generate very low frequency difference signals when applied to the logic circuit. This can adversely affect the operation of the logic circuit for the duration of time that the disturbance signal persists. For example, if the difference in RC time constants between the right and left channel integrators is 1/10th Hz, this can generate a 0.1 Hz disturbance signal having a persistence of several seconds. During this interval, the logic circuit is incapable of accurately measuring the time difference between the signals generated by the Coriolis effect meter.
It is therefore seen that there currently exists a need for improved circuitry for canceling the DC component that may appear on the input of an AC integrator so that the integrator may operate as a high pass filter for a received AC signal.